1. Field of the Invention
This invention relates to an adhesive for electroless plating having excellent thermal resistance, electrical insulation resistance, chemical stability and adhesion property to electroless plated film as well as a printed circuit board using such an adhesive and a method of producing the same.
2. Related Art Statement
Recently, the advancement of electronics is remarkable, and as a result it is attempted to further miniaturize electronic equipment and achieve high speed access of the function. For this purpose, printed circuit boards, particularly printed circuit boards provided with parts such as IC, LSI and the like, are required to have high densification and reliability through fine patterns.
Heretofore, there is typically employed an etched foil method wherein a copper foil is laminated onto a substrate and then etched, as a method of forming a conductor circuit on the printed circuit board. According to this method, the conductor circuit having an improved adhesion property to the substrate can be formed, but since the copper foil is thick, it is difficult to obtain a fine pattern of high accuracy by the etching, and also there are further problems that the production steps are complicated, and the efficiency is bad, and the like.
Lately, therefore, there is proposed an additive method wherein an adhesive layer is formed on the surface of the substrate. The surface of the adhesive layer then toughened and subjected to electroless plating to form a conductor circuit as the method of forming the conductor circuit on the printed circuit board.
As the adhesive used in this additive method, there are known an adhesive containing diene series synthetic rubber, an adhesive disclosed in U.S. Pat. No. 4,216,246 and the like. However, the former adhesive containing the synthetic rubber has drawbacks that the adhesion strength becomes much lower at high temperatures, and the electroless plated film swells during soldering, and the like. Further, the thermal resistance is low and electric properties, such as surface resistance and the like, are insufficient, so that the use region is fairly restricted. On the other hand, in the adhesive as disclosed in U.S. Pat. No. 4,216,246, a thermosetting resin component constituting the spherical granule for the formation an anchor in the adhesive is not etched, and such spherical granule for the formation the anchor is simultaneously cured together with a heat-resistant resin as a matrix, so that it is difficult to control the shape and size of the granule. Further, the toughened surface of the adhesive layer formed on the substrate of a relatively uneven roughness, so that it is also difficult to obtain a conductor circuit of fine pattern.
Lately, printed circuit boards in which wiring circuits are formed at a multilayer state have been used for high densification in the printed circuit board.
As the printed circuit board having the multilayered wiring circuits, there has hitherto been used a printed circuit board obtained, for example, by laminating plural circuit plates, each provided with an internal circuit pattern, one upon the other through a prepreg as an insulating layer, pressing them, forming a through-hole therein and then connecting and electrically conducting the internal circuit patterns to each other through the through-hole.
In the aforementioned printed circuit board, however, plural internal circuit patterns are connected and electrically conducted to each other through the through-holes formed therein, so that it is difficult to realize high densification or high speed access by the formation of a complicated wiring circuit pattern.
As a printed circuit board capable of overcoming the above difficulty, there has been developed progressed to develop printed circuit boards wherein conductor circuits and organic insulating films are alternately built up on each other. Such a printed circuit board is suitable for extra-high densification and high speed access, but it is difficult to form an electroless plated film onto the organic insulating film with a good reliablity. In this printed circuit board, therefore, the conductor circuit is formed by PVD process such as vapor deposition, sputtering and the like, or by using electroless plating together with the above PVD process. However, the method of forming a conductor circuit through the PVD process is poor in productivity and undesirably high in cost.
The inventors have made various studies for solving the drawbacks in the adhesive for electroless plating and the printed circuit board as mentioned above, and have already proposed solutions in U.S. Pat. No. 4,752,499 and Japanese Patent laid open No. 63-126,297.
However, the above proposed technique still has a problem that when there is no great difference in the solubility to a particular chemical between the particulate substance and the matrix resin, the resulting anchor is apt to become unclear, and consequently the adhesion property of the plated film is not improved.